This invention relates to compositions and processes for forming on metals, particularly aluminum and its alloys, a conversion coating that is substantially transparent, so that aesthetically appealing characteristics of the metal surface that is conversion coated are not overly impaired by the conversion coating. (No quantitative test is in common use to measure the degree of preservation of the aesthetic qualities of the metal, but for the principal application area of this invention, aluminum beverage cans, any ordinary consumer can readily judge whether the bright reflectance of the can is as good as usual.)
The following discussion and the description of the invention will be set forth primarily for aluminum cans. However, it is to be understood that, with any obviously necessary modifications, both the discussion and the description of the invention apply also to tin plated steel cans and to other types of metal surfaces over which a transparent conversion coating is practically interesting.
Aluminum cans are commonly used as containers for a wide variety of products, particularly beverages. The exterior cylindrical surfaces of such cans normally are at least partially decorated with lacquer and/or printing ink and the interior surfaces, including the inner dome, normally are protected with sanitary lacquer, but the outer domes of the cans usually do not have lacquer or any similar coating, except possibly for a xe2x80x9crim coatxe2x80x9d on its outer margin. It is considered desirable in the market for all exterior parts of the can to have a lustrous surface, even in the parts that are printed or colored, with the reflective properties that are essentially unique to polished metal surfaces. Therefore, for this particular field of use, the conversion coated surface must have adequate adhesion to printing inks and/or lacquer.
In the most widely used current commercial practice, at least for large scale operations, aluminum cans are typically subjected to a succession of six cleaning and rinsing operations as described in Table A below. It is preferable to include another stage, usually called xe2x80x9cPrerinsexe2x80x9d, before any of the stages shown in Table A; when used, this stage is usually at ambient temperature (i.e., 20-25xc2x0 C.) and is most preferably supplied with overflow from Stage 3 as shown in Table A and is next most preferably supplied with overflow from Stage 1 as shown in Table A. The prerinse may also be tap water. Any of the rinsing operations shown as numbered stages in Table A preferably includes two, or more preferably three, sub-stages, which in consecutive order of their use are usually named xe2x80x9cdrag-outxe2x80x9d, xe2x80x9crecirculatingxe2x80x9d, and xe2x80x9cexitxe2x80x9d or xe2x80x9cfresh
waterxe2x80x9d sub-stages; if only two sub-stages are used, the name xe2x80x9cdrag-outxe2x80x9d is omitted. Most preferably, when such sub-stages are used, a blow-off follows each sub-stage, but in practice such blow-offs are often omitted. Also, any of the stages numbered 1 and 4-6 in Table A may be omitted in certain operations.
A conversion coating over the metallic surfaces of beverage cans, prior to any lacquer coating or printing, is generally considered desirable, in order to increase the adhesion of the inner sanitary lacquer and exterior decorative and protective coatings, especially when the cans in process are to be subjected to stressful metal working operations such as necking (i.e., reducing the can diameter in its neck region) and flanging (to provide an anchoring point for a separate cap for the filled container). However, prior art conversion coatings that were sufficiently transparent to preserve the metallic luster of the surfaces coated with them were susceptible to at least one of the following disadvantageous characteristics when subjected to subsequent heating: development of exterior dome staining during pasteurization; loss of luster of the exterior dome surface upon its exposure to steam; and substantially weakened adhesion of lacquers and printing inks, compared with surfaces of otherwise identical aluminum having the same conversion coating not exposed to heat and sometimes even compared to otherwise identical aluminum that never had any conversion coating. A major object of the present invention is to provide a transparent conversion coating that will avoid or reduce at least one, or preferably all, of these disadvantageous characteristics of the prior art. Other objects will be apparent from the further description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word xe2x80x9caboutxe2x80x9d in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, xe2x80x9cparts ofxe2x80x9d, and ratio values are by weight or mass; the term xe2x80x9cpolymerxe2x80x9d includes xe2x80x9coligomerxe2x80x9d, xe2x80x9ccopolymerxe2x80x9d, xe2x80x9cterpolymerxe2x80x9d and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word xe2x80x9cmolexe2x80x9d means xe2x80x9cgram molexe2x80x9d, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; the terms xe2x80x9csolutionxe2x80x9d, xe2x80x9csolublexe2x80x9d, xe2x80x9chomogeneousxe2x80x9d, and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25xc2x0 C.; and the first definition of an acronym or other abbreviation applies to all subsequent uses of the same acronym or other abbreviation.
In accordance with this invention, it has been found that the major object as stated above is achievable with an aqueous conversion coating composition that comprises, preferably consists essentially of, or more preferably consists of, water and the following components:
(A) a component of one or more dissolved transition metal compounds that contains zirconium, hafnium, or both (hereinafter, for brevity, only zirconium will be mentioned, but it is to be understood that zirconium may be partially or totally replaced by hafnium) and may also include titanium, provided that zirconium constitutes at least 30% of the moles of the total moles of zirconium and titanium;
(B) a component of at least one dissolved compound that contains inorganically bonded fluorine and is not part of component (A); and
(C) a component of dissolved organic polymer molecules; and, optionally, one or more of the following components:
(D) a component of dissolved chelating agent molecules that are not part of any of immediately previously recited components (A) through (C);
(E) a component of dissolved acid molecules that are not part of any of immediately previously recited components (A) through (D);
(F) a component of preservative molecules that are not part of any of immediately previously recited components (A) through (E); and
(G) a component of dissolved cations from the metal surface being or to be conversion coated.
Various embodiments of the invention include a liquid working composition that will form a conversion coating upon contact with a suitable metal substrate for a sufficient period of time; a concentrate composition that when mixed with water, and optionally with other materials, will form a liquid working composition, a process including forming a substantially transparent conversion coating on a metal surface by contacting the metal surface with such a working composition, and a metal article having a conversion coating formed according to the invention on at least part of its surface.
Phosphate is one of the most common ingredients of conversion coatings in general. However, a working composition according to this invention preferably does not contain more than, with increasing preference in the order given, 1.0, 0.5, 0.3, 0.10, 0.050, 0.020, 0.0100, 0.0050, 0.0020, 0.0010, 0.00050, 0.00020, or 0.00010 percent of phosphorus atoms contained within any dissolved oxyacids of phosphorus, or any partially or completely neutralized salts of such acids, that are dissolved in the composition, because it has been found that, if the concentration of phosphorus containing inorganic anions in a working composition according to the invention is too high, a hazy white stain usually develops on a beverage container treated with the working composition and later exposed to steam.
The zirconium that is required for component (A) and the titanium that is permitted as part of component (A) may be supplied by any water soluble compound of these two metals, which may be dissolved in either cationic or anionic form. At least for economy, because it reduces the amount of component (B) that is needed, hexafluorozirconic acid and its salts are the preferred sources of zirconium and hexfluorotitanic acid and its salts are the preferred source of titanium, with the acids independently most preferred in both instances. In a working composition according to the invention, the total concentration of dissolved zirconium and titanium atoms preferably is at least, with increasing preference in the order given, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, or 0.88 millimoles per liter of working composition, a unit of concentration that may be freely applied hereinafter to other constituents as well as to zirconium and titanium and is hereinafter usually abbreviated as xe2x80x9cmM/lxe2x80x9d. Of this total, more than 30 mole % must be zirconium, and preferably, with increasing preference in the order given, at least 35, 40, 45, or 50 mole percent is zirconium. There is no technical disadvantage if the only transition metal atoms in component (A) are zirconium.
Considerably larger concentrations of zirconium and, optionally, titanium may be used without significant technical disadvantage. For example, more than 4 mM/l of hexafluorozirconic acid may be used without substantial deterioration in performance of the conversion treated containers in any of the tests described in the examples. However, there is somewhat increased danger of separation of the working composition into two phases and there is no improvement in performance. Therefore, at least for economy, the total concentration of zirconium and titanium in a working composition according to the invention preferably is not more than, with increasing preference in the order given, 3.5, 3.0, 2.5, 2.0, 1.8, 1.6, 1.4, 1.2, or 1.0 mM/l.
Irrespective of the source(s) of zirconium and, optionally, titanium, a working composition according to the invention preferably contains more than enough dissolved fluorine atoms in anionic form to form ZrF6xe2x88x922 anions with all of the zirconium dissolved in the working composition and TiF6xe2x88x922 anions with any titanium present. For this reason, as already noted, supplying these metals in the form of salts or acids that contain these anions is normally preferred. However, solutions that contain hexafluorozirconate with no additional fluoride are susceptible to precipitation of the zirconium as basic salts. Therefore, the molar ratio of fluorine to the total of zirconium and titanium in a working composition according to the invention preferably is at least, with increasing preference in the order given, 6.05:1.00, 6.10:1.00, 6.15:1.00, 6.20:1.00, or 6.25:1.00. Component (B) preferably therefore contains at least a sufficient amount of soluble anionic fluoride to achieve at least one of these ratios in the working composition, when considered together with any fluorine present in component (A). Sufficient hydrofluoric acid to achieve one of these ratios is often added by commercial suppliers of hexafluorozirconic and hexafluorotitanic acids. If these materials are used as the sources for component (A), it may not be necessary to include any additional source for component (B). If such an additional source is needed, hydrofluoric and/or tetrafluoroboric acids are preferred, as considered further below.
Soluble anionic fluorine has at least two other important effects in a working composition according to the invention: It promotes the dissolution of aluminum atoms, which at least at the instant of dissolution are believed to be trivalent cations, from the surface to be conversion coated, and, to the extent stoichiometrically and thermodynamically possible, it forms coordinate complexes with the aluminum so dissolved or any other aluminum cations introduced into the solution. Only fluoride ions that are not already bound into complex ions and/or exist in equilibrium from dissociation of complex ions are effective in promoting dissolution of aluminum cations and complexing them. The effective concentration of this kind of fluoride, called xe2x80x9cfree fluoridexe2x80x9d herein and xe2x80x9cavailable fluoridexe2x80x9d in U.S. Pat. No. 4,273,592, is defined and can conveniently be measured by means of a fluoride sensitive electrode as described in U.S. Pat. No. 3,431,182 and commercially available from Orion Instruments. xe2x80x9cFree fluoridexe2x80x9d as this term is used herein was measured relative to xe2x80x9cStandard Solution 120 MCxe2x80x9d commercially available from the Henkel Surface Technologies (xe2x80x9cHSTxe2x80x9d) Division of Henkel Corporation, Madison Heights, Mich. by a procedure described in detail in HST Technical Process Bulletin No. 1580. The Orion Fluoride Ion Electrode and the reference electrode provided with the Orion instrument are both immersed in the noted Standard Solution and the millivolt meter reading is adjusted to 0 with a digital keypad on the instrument, after waiting if necessary for any initial drift in readings to dissipate. The electrodes are then rinsed with deionized or distilled water, dried, and immersed in the sample to be measured, which should be brought to the same temperature as the noted Standard Solution had when it was used to set the meter reading to 0. The reading of the electrodes immersed in the sample is taken directly from a millivolt (hereinafter usually abbreviated as xe2x80x9cmvxe2x80x9d) meter on the instrument.
Free fluoride is preferably supplied to a working composition according to the invention as HF or soluble neutral or acid salts thereof, and the amount of it preferably is such as to give an electrical potential of a fluoride sensitive electrode in contact with the working composition that is at least, with increasing preference in the order given, xe2x88x92200, xe2x88x92170, xe2x88x92150, xe2x88x92140, xe2x88x92130, xe2x88x92120, xe2x88x92110, xe2x88x92105, xe2x88x92100, xe2x88x9295, or xe2x88x9291 mv, compare with the potential of the same fluoride sensitive electrode in contact with the Standard Solution noted above, and independently preferably is not more than, with increasing preference in the order given, 100, 70, 40, 10, 0, xe2x88x9210, xe2x88x9220, xe2x88x9230, xe2x88x9240, xe2x88x9250, xe2x88x9260, xe2x88x9270, xe2x88x9275, xe2x88x9280, xe2x88x9285, or xe2x88x9289 mv. If the working composition has a potential of less than xe2x88x92200 mv, the aluminum surface being conversion coated will usually be etched too rapidly to retain a lustrous appearance as desired and may even fail to form any of the desired conversion coating, while if the working composition has a potential of more than 100 mv, the rate of formation of the conversion coating usually will be impractically slow, and the working composition is likely to form precipitates on standing. Free fluoride thus constitutes at least part of component (B) of a composition according to the invention.
Although the amount of free fluoride must be limited for the reasons given above, a xe2x80x9creservoirxe2x80x9d of fluoride is advantageously present in a working composition according to the invention, so that the concentration of dissolved aluminum cations in the bulk of the working composition, as contrasted to the immediate vicinity of the surface being conversion coated, can not become so large as to cause difficulties in the conversion coating process. Such a reservoir of fluoride is conveniently and preferably provided by including in a working composition according to the invention, as part of its component (B), complex anions including fluorine that have a higher dissociation equilibrium constant than hexafluorozirconate but not so high as to contribute an excessive amount of free fluoride. Tetrafluoroboric acid and its salts are preferred for this purpose, the acid being more preferable at least for economy. Irrespective of the source, 20 a working composition according to the invention preferably includes dissolved tetrafluoroborate in a concentration that is stoichiometrically equivalent to a concentration of tetrafluoroboric acid that is at least, with increasing preference in the order given, 0.005, 0.010, 0.020, 0.030, 0.035, 0.040, 0.045, 0.047, 0.049, or 0.051 parts by weight per thousand parts of total composition, this concentration unit being hereinafter usually abbreviated as xe2x80x9cpptxe2x80x9d, and independently preferably is not more than, with increasing preference in the order given, 0.5, 0.30, 0.20, 0.15, 0.10, 0.080, 0.075, 0.070, 0.065, 0.060, or 0.055 ppt. Fully satisfactory results can be obtained from freshly made working compositions that do not include any such free fluoride reservoir, but the presence of such a reservoir promotes performance stability after substantial amounts of aluminum have been dissolved into the working composition by prolonged use.
In addition to providing a reservoir of fluoride that can become free as free fluoride originally in the composition is consumed, tetrafluoroborate is believed to have a preservative effect because of its boron content. This can be advantageous to the storage stability of compositions according to the invention, because some of the organic ingredients in these compositions are capable of nourishing microorganisms that may enter them from various ambient environments.
Component (C) as described above is preferably selected from the group of water soluble polymers that contain, in each molecule, at least one of two types of polar moieties: (1) acidic moieties such as carboxylate, phosphonate, sulfate, and the like, which are independently preferably neutralized with a strong alkali, in order to maximize the degree of localization of electrically negative charges in the vicinity of these acidic moieties within the polymers that contain them, when these polymers are dissolved in the at least mildly acidic preferred compositions according to the invention; and (2) nucleophilic moieties such as amino nitrogen, phosphino phosphorus, and the like that form localized positive charge centers, when dissolved in preferred compositions according to the invention, by attracting protons to themselves to form cationic moieties. More particularly, component (C) as described above preferably contains at least one of:
a sufficient number of acidic moieties that the number ratio of acid moieties to total carbon atoms within component (C) is at least, with increasing preference in the order given, 1.0:9.0, 1.0:8.0, 1.0:7.0, 1.0:6.0, 1.0:5.0, 1.0:4.0, 1.0:3.0, or 1.0:2.0; and
a sufficient number of nucleophilic moieties that the number ratio of nucleophilic moieties to total carbon atoms within component (C) is at least, with increasing preference in the order given, 1.0:50, 1.0:40, 1.0:30, 1.0:27, 1.0:24, 1.0:22, 1.0:20, 1.0:18, or 1.0:16.
In a first particularly preferred embodiment of the invention, component (C) comprises polymers that contain, as at least 10% of their total mass, one or more aminomethylphenyl moieties, each of which conforms to the following general formula: 
wherein:
each of R1 through R3 is selected, independently of each other and independently from one aminomethylphenyl moiety of the component to another, from the group consisting of a hydrogen moiety, an alkyl moiety with from 1 to 5 carbon atoms, and an aryl moiety with from 6 to 18 carbon atoms;
each of Y1 through Y3 is selected, independently of each other and independently from one aminomethylphenyl moiety to another, from the group consisting of: a hydrogen moiety; a xe2x80x94CH2Cl moiety; an alkyl moiety with from 1 to 18 carbon atoms; an aryl moiety with from 6 to 18 carbon atoms; a moiety conforming to the general formula xe2x80x94CR12R13OR14, where each of R12 through R14 is selected from the group consisting of a hydrogen moiety, an alkyl moiety, an aryl moiety, a hydroxyalkyl moiety, an aminoalkyl moiety, a mercaptoalkyl moiety, and a phosphoalkyl moiety; and a moiety Z that conforms to one of the two immediately following general formulas: 
xe2x80x83wherein:
each of R5 through R7 is selected, independently of each other and independently from one aminomethylphenyl moiety to another, from the group consisting of a hydrogen moiety, an alkyl moiety, an aryl moiety, a hydroxyalkyl moiety, an aminoalkyl moiety, a mercaptoalkyl moiety, and a phosphoalkyl moiety; R8 is a polyhydroxy alkyl moiety and R9 is selected from the group consisting of a hydrogen moiety, an alkyl moiety, an aryl moiety, a hydroxy or polyhydroxy alkyl moiety, an amino or polyamino alkyl moiety, a mercapto or polymercapto alkyl moiety, a phospho or polyphospho alkyl moiety, an xe2x80x94Oxe2x88x92 moiety, and an xe2x80x94OH moiety;
Y4 is a moiety Z as above defined; and
W1 is selected, independently from one molecule of the component to another and from one to another aminomethylphenyl moiety, from the group consisting of a hydrogen moiety, an acyl moiety, an acetyl moiety, a benzoyl moiety; a 3-allyloxy-2-hydroxypropyl moiety; a 3-benzyloxy-2-hydroxypropyl moiety; a 3-butoxy-2-hydroxypropyl moiety; a 3-alkyloxy-2-hydroxypropyl moiety; a 2-hydroxyoctyl moiety; a 2-hydroxyalkyl moiety; a 2-hydroxy-2-phenylethyl moiety; a 2-hydroxy-2-alkylphenylethyl moiety; a benzyl, methyl, ethyl, propyl, unsubstituted alkyl, unsubstituted allyl, or unsubstituted alkylbenzyl moiety; a halo or polyhalo alkyl, or halo or polyhalo alkenyl, moiety; a moiety derived from a condensation polymerization product of ethylene oxide, propylene oxide or a mixture thereof by deleting one hydrogen atom therefrom; and a sodium, potassium, lithium, ammonium or substituted ammonium, or phosphonium or substituted phosphonium cation moiety.
When component (C) is selected so as to correspond to this first particularly preferred embodiment:
its molecules preferably have a weight average molecular weight that is at least, with increasing preference in the order given, 360, 700, 1500, 3000, 6000, or 10,000 daltons and independently preferably is not more than, with increasing preference in the order given, 2,000,000, 1,000,000, 500,000, 250,000, 125,000, 70,000, or 30,000 daltons; and, independently,
the concentration of component (C) in a working composition according to the invention preferably is at least, with increasing preference in the order given, 0.009, 0.018, 0.030, 0.045, 0.060, 0.075, or 0.085 grams per liter (hereinafter usually abbreviated as xe2x80x9cg/lxe2x80x9d) and independently preferably is not more than, with increasing preference in the order given, 0.36, 0.30, 0.24, 0.21, 0.18, 0.15, or 0.12 g/l.
Still more preferably within this first particularly preferred embodiment, component (C) is selected from molecules that include, as at least, with increasing preference in the order given, 10, 20, 30, 40, 50, 60, 70, 80, or 90% of their total mass, aminomethylphenyl moieties that conform to the general formulas for such moieties given above when, independently for each part of the general formulas stated: each of R1 through R3, R5, R6, Y1 through Y3, and W1 is a hydrogen moiety; R7 is an alkyl moiety or a hydrogen moiety, preferably an alkyl moiety having not more than, with increasing preference in the order given, 5, 4, 3, 2, or 1 carbon atoms; R8 is a moiety conforming to the general formula xe2x80x94CH2(CHOH)pxe2x80x94H, where p is an integer that is at least, with increasing preference in the order given, 2, 3, 4, or 5 and independently preferably is not more than, with increasing preference in the order given, 12, 10, 8, 7, or 6; and R9 is not present (or, in other words, Z conforms to the left rather than to the right one of its two alternative general formulas as above shown).
In a second particularly preferred embodiment, component (C) is selected from polymers of at least one of maleic, acrylic, and methacrylic acids. For this second particularly preferred embodiment:
the concentration of component (C) in a working composition according to the invention preferably is at least, with increasing preference in the order given, 0.009, 0.015, 0.030, 0.035, 0.040, 0.045, 0.050, 0.055, 0.060, or 0.064 g/l and independently preferably is not more than 1.0, 0.8, 0.6, 0.40, 0.30, 0.20, 0.15, or 0.10 g/l, these maximum preferences being primarily for economy; and, independently,
the molecular weight of the polymer preferably is at least, with increasing preference in the order given, 200, 300, 400, 500, 600, 700, 800, 900, or 1000 and independently preferably is not more than, with increasing preference in the order given, 107, 106, 105, 8xc3x97104, 6xc3x97104, 4xc3x97104, 2xc3x97104, or 1.0xc3x97104.
Although not necessary, the presence of optional component (D) of chelating agents in a composition according to the invention is usually preferred, at least when the composition contains amounts of dissolved calcium and/or magnesium ions that are characteristic of hard domestic and industrial water supplies. Any chelating agent present is preferably selected from the group consisting of molecules each of which contains at least two nucleophilic moieties selected from the group consisting of carboxyl, carboxylate, non-carboxyl hydroxyl, amino, thio, phosphonic acid, phosphonate, phosphinic acid, and phosphinate moieties, with these at least two moieties being bonded into the molecule in positions such that a five- or six-membered ring of atoms can be formed by atoms in the molecule and a multivalent metal atom that is coordinatively covalently bonded to a nucleophilic atom (oxygen, nitrogen, sulfur, or phosphinic phosphorus) in each of said nucleophilic moieties in the molecule. More preferably, any chelating agent is selected from the group consisting of gluconic, citric, tartaric, and malic acids, and water soluble salts of all of these acids, or still more preferably from gluconic acid and its salts. Independently of its specific chemical nature, the concentration of chelating agents in a working composition according to the invention preferably is at least, with increasing preference in the order given, 0.020, 0.040, 0.060, 0.080, 0.10, 0.12, 0.14, or 0.16 mM/l, and independently at least for economy, preferably is not more than, with increasing preference in the order given, 2, 1.0, 0.8, 0.60, 0.50, 0.40, 0.35, 0.30, 0.25, or 0.20 mM/l.
A working composition according to the invention preferably has a pH value that is at least, with increasing preference in the order given, 1.0, 1.5, 2.0, 2.5, 2.7, 2.9, or 3.1 and independently preferably is not more than, with increasing preference in the order given, 5.0, 4.5, 4.3, 4.1, 3.9, 3.7, 3.5, or 3.3. When other ingredients of a composition according to the invention are near their most preferred values, achieving this pH value will normally require an acid that does not contain fluorine, and in such instances, nitric acid is preferred, although other acids such as sulfuric that do not contain phosphorus may alternatively be used as optional component (E). If an initial concentration of aluminum is desired as considered further below, aluminum salts of one of these acids may conveniently be used to provide both constituents.
A preservative agent, optional component (F), may be needed in some environments to protect against growth of microorganisms in a stored composition according to the invention. Numerous suitable preservatives are known to those skilled in the art and may be utilized in such instances.
As already noted, a working composition according to the invention will dissolve some aluminum from a surface that it is conversion coating. If the working composition contains essentially no dissolved aluminum at the beginning of its use, some of the dissolved aluminum will not be incorporated into the conversion coating, but instead will accumulate in the solution to constitute optional component (G). Because of this, it is often preferred to add optional component (G) at the beginning, in order to reduce the possibility of excessive etching of the aluminum surface being conversion coated with a freshly made, aluminum-free working composition. If this is desired, water soluble aluminum salt(s) of strong, phosphorus-free acids are most preferably used for the purpose. Unless fresh working composition is supplied at a high rate that is usually not economically justifiable, component (G) will eventually accumulate to a steady state value that is usually at least 0.1 g/l, and may be as high as 0.75 g/l, of dissolved aluminum. Provided that the concentration of free fluoride is kept within the preferred range given above, this amount of component (G) has no harmful effect on the conversion coating process.
In a make-up concentrate composition according to the invention, the concentrations of the various components identified above preferably are considerably higher than those preferred for working compositions. More specifically, in a make-up concentrate according to the invention, independently for each component stated, the concentration of each of those of components (A) through (G) that are present in the make-up concentrate preferably corresponds to at least, with increasing preference in the order given, 10, 30, 50, 70, 90, 100, 150, 200, 300, 400, or 500 times the preferred values specified above for the same component in a working composition. Greater concentrations in the make-up concentrate are more economical, because of the reduced cost of shipping water that can readily be added at the point of use, but lower concentrations are less susceptible to phase separations during storage. Although sometimes such phase separations are harmless if the entire concentrate is thoroughly mixed before being used, separations certainly increase the risk of not obtaining the intended concentrations of every ingredient when preparing a working composition from the concentrate. The pH and free fluoride values do not scale linearly with concentration as do the concentrations of ingredients such as zirconium and polymer, so that the preferred values for concentrates for these characteristics are those that will give the preferred pH and free fluoride values when diluted so as to provide the concentrations preferred for zirconium and polymer.
In a process according to the invention, the temperature of a working composition as described above preferably is maintained during its contact with the metal surface to be conversion coated at a temperature that is at least, with increasing preference in the order given, 30, 32, 34, 36, 38, 40, 42, 44, 46, or 48xc2x0 C. and independently, primarily for economy, preferably is not more than, with increasing preference in the order given, 75, 65, 60, 58, 56, 54, 52, or 50xc2x0 C. At least one temperature within this range will normally achieve formation of an effective conversion coating within a contact time of from 2 to 50 seconds, as required by most existing commercial container manufacturing and decorating plants at their current and/or projected line speeds.